1. Field of the Invention
The present invention relates to a magnetic recording medium incorporating a thin magnetic metal film. More particularly, the present invention relates to a magnetic recording medium for use as a video tape for performing a recording operation for a long time or a tape streamer having a large capacity.
2. Description of the Related Art
Hitherto, a magnetic recording medium in the form of a thin metal magnetic film incorporates a support member which is usually made of polyethylene terephthalate film. The reason for this lies in that the polyethylene terephthalate film has high strength and satisfactory dimension stability. In particular, a polyethylene terephthalate film having a thickness of about 7 xcexcm to 10 xcexcm is employed to form a home video tape, for example, 8 mm tape. A polyethylene terephthalate film having a thickness of about 5 xcexcm to 7 xcexcm is employed to form a tape streamer.
In recent years, the size of a DVC (Digital Video Cassette) tape, which is one of video cassette tapes, has been reduced. Also further long time recording is required. Enlargement of the capacity of the tape streamer, for example, D8 (data-8 mm) and DDS (Digital Data Storage), has been required.
Moreover, use under wide environment conditions (in particular, temperature and humidity conditions which are frequently changed), reliability in data preservation, stable recording and reproduction of data even in repeated use at high speed have been required.
To realize long time recording and enlargement of the capacity of the magnetic recording medium, the thickness of the base film must be reduced. When the thickness is reduced, there arises a problem of decrease in the stiffness and deterioration in the skew characteristic.
To realize required stiffness and the skew characteristic, contradictory characteristics, which are high strength and prevention against thermal shrinkage, must simultaneously be satisfied. Therefore, the strength of the polyethylene terephthalate film has been increased by performing re-orientation or the like. Moreover, aging has been performed to prevent thermal shrinkage. Since the thickness of the film has furthermore been reduced in recent years, the strength realized by the polyethylene terephthalate film has limitation. As a base film for a next-generation high density magnetic recording medium, an aromatic polyamide film has attracted attention because the aromatic polyamide film is free from considerable thermal shrinkage.
Since the recording time has been elongated and the capacity has been enlarged, the magnetic recording medium must meet furthermore severe requirements. That is, the magnetic recording medium must has a further improved electromagnetic conversion characteristic. Moreover, further reduction in the error rate has been required.
In view of the foregoing, it is preferable that the base film has a flat mirror surface which is free from any projection. An influence of the surface characteristic of the base film is, however, easily exerted on the surface characteristic of an evaporate film formed on the base film. Therefore, surface of the evaporated film formed on the base film maintains the mirror characteristic. Hence it follows that great friction occurs with respect to the magnetic head, causing the smooth movement of the evaporated surface to deteriorate. As a result, there arises a problem in that damage is sustained and the error rate is raised due to dust separated from the evaporated film.
In view of the foregoing, an object of the present invention is to provide a magnetic recording medium exhibiting both satisfactory electromagnetic conversion characteristic and durability against movement even if an aromatic polyamide film is employed to serve as the base film.
To achieve the foregoing object, according to one aspect of the invention, there is provided a magnetic recording medium comprising: a non-magnetic support member having a principal plane on which a magnetic layer constituted by at least a thin metal film is formed, wherein the non-magnetic support member has the principal plane on which at least the thin metal film is formed and on which small projections, the height of each of which is 10 nm to 40 nm from the central surface, are formed at a density of 1,000,000 pieces/mm2 to 20,000,000 pieces/mm2, and a bearing curve of the principal plane of the non-magnetic support member on which the small projection have been formed is such that the gradient of a straight line is xe2x88x920.3 or smaller when a range of 10 nm in the upper portion is linearly approximated with a straight line by a least square approximation and the intercept is 10 nm to 40 nm.
The magnetic recording medium according to the present invention and structured as described above is arranged such that the density of the small projections which are formed on the principal plane of the non-magnetic support member, and the height of each of which from the central surface is 10 nm to 40 nm is specified. Moreover, the bearing curve of the principal plane of the non-magnetic support member on which the small projections have been formed is such that the gradient of a straight line realized when the range of 10 nm in the upper portion is approximated with a straight line by the least square approximation and the intercept are specified. The thin magnetic metal film is formed on the non-magnetic support member. Therefore, the principal plane of the thin magnetic metal film of the magnetic recording medium has a required flatness.
The bearing curve will now be described. The cross sectional shape of the surface is measured to produce a cross sectional curve indicating the cross sectional shape. Then, a portion of the cross sectional curve (having reference length E) is cut. The cut portion is cut along a cut line which is lowered from the vertex for a predetermined distance. The sum of the cut length of the cross sectional curve cut with the cut line is divided with the reference length E. The thus-obtained value for each cut line is indicated with percentage. The obtained curve is the bearing curve.
The central surface will now be described. A surface set such that the sum of squares of deviations of the distances from a rough curved surface is minimized is an average surface. When a cross section which is in parallel with the average surface of the rough curved surface has been formed, a surface having the areas each of which is enclosed by the foregoing cross section and the rough curved surface and which are the same between the two side portion of the cross section is the central surface.
The density of the small projections has been measured by using a so-called atomic force microscope (AFM). Specifically, an atomic force microscope (AFM) manufactured by Digital Instruments was used.
Other objects, features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.